I have to say I am very surprised there is room for the ridiculous door pocket, the toilet and the corridor there. I'm also disappointed that the rubbish pressure pad doors are still there.
At least initially, the GWR project was conceived as a short-term stop-gap to make up for the need to retain more Turbo sets than expected in the Thames Valley area as the result of electrification delays, which required 158s intended for the Devon-Cornwall regional service to stay on other jobs - hence the GWR project does not involve major work beyond the installation of the sliding doors and toilet tanks.
Chiltern plug doors.
EDIT: Also, what's the point of pocket doors on an EXPRESS passenger train when they were designed to speed up boarding on STOPPING trains. IMO it looks daft and out of place. Plug doors also keep the lines clean rather than the break-up of them on the Super Hitachi Intercity Trains.
The Chiltern 'conversions' actually involved cutting off and rebuilding both ends of each coach - so ever so slightly more expensive than the insertion of the sliding doors now being carried out.
Have you ever observed how much time is spent at station stops on GWR HSTs, even on shorter platforms, checking slam doors are shut? Sliding doors will make the lives of the guards and platform staff (where available) a sight more straightforward. Press a button, doors close and lock, indicator lights go out and away you go.